This invention relates generally to Computed Tomography (CT) imaging systems, and more particularly, to target angle heel effect compensation.
In at least some known imaging systems, an x-ray tube source projects an x-ray beam which passes through an object being imaged, such as a patient, and impinges upon an array of x-ray detector rows. This technique is quite effective in medical CT scanners, but it has some drawbacks when the detector coverage becomes large, as in the case of multi-slice CT. With the advent of multi-slice CT imaging systems including a plurality of detector rows, at least two major drawbacks exist, a non-uniform x-ray flux and a non-uniform slice thickness. The non-uniform x-ray flux may result in a heel effect and a non-uniform slice thickness may result in variations in the spatial resolution.
The effects of the heel effect can produce image quality differences over the detector rows. For example, a 40 mm Volumetric Computed Tomography (VCT) detector with a nominal 7 degree target angle has an effective target angle of 5 degrees on the outer anode side row and 9 degrees on the outer cathode side row, resulting in an intensity variation of roughly 20% from one end of the detector to the other. This variance in radiation intensity due to the heel effect reduces image quality over the x-ray detector rows, and therefore reduces the image quality of the radiographs.
Non-uniform slice thickness results when a first projected focal spot height is significantly larger that a second projected focal spot height. Non-uniform slice thickness translates to a spatial resolution in the z-axis becoming a function of detector row.
In multi-slice CT, it is desirable to design a system such that both the x-ray flux and the spatial resolution do not change significantly from detector row to row.